also refactor tests.nix and default.nix so they share more of the environment
setup which is now slightly more complex

the branch has some python packaging fixes in nixpkgs that we need

there will be tons of pyflakes warnings from the python3 porting strategy

Refactor the Nix packaging for reduced duplication and to provide a more useful shell environment

it is nice to fail if someone passes in something that is not supposed to be
passsed in, rather than silently accepting and ignoring it

meh this is not great but tests.nix passes through a bunch of its args that
get captured in the args set... probably need to write and use set subtraction
to get them out, I guess.  First, see if this works though.

Depend on/Compatibility with Tahoe-LAFS 1.17.0

We only have one version of tahoe-lafs here right now anyway, so the
identifier isn't ambiguous without a version number, and there are probably
more changes required to really support more than one version at a time.